1. Field of the Invention
The present invention in a broad aspect relates to inhibiting the corrosion of metals. The invention more particularly concerns compositions and methods of controlling the pH proximate a point of incipient corrosion of a metal at a pH value where the metal is passive to corrosion. The invention more especially concerns coating compositions for metals that provide pH control at a metal surface for passivation. The coating compositions are water vapor permeable and comprise a reservoir layer that is preferably of a water repellent but semipermeable carrier in which one or more particulate or liquid passivating materials are dispersed. The passivating materials preferably include one or more particulate or liquid pH buffers, but can also include other types of corrosion preventative materials, such as corrosion inhibitors. The invention can take the form of a single layer coating or of a coating formed by a plurality of layers.
2. Related Art
The corrosion of steel and other metal products continues to be a serious technical problem which has profound effects an the economy and the standard of living. It causes premature replacement of infrastructure, which in turn causes loss of natural resources, and gives rise to inferior roads and buildings. It also causes premature replacement of equipment and parts in industry and in boats and other marine vehicles, automobiles, and aircraft.
The process of corrosion requires several physical conditions. These conditions include a metallic path, an electrolyte, anode, cathode, and a potential difference between the anode and the cathode (tendency to corrode). A metallic path allows the transfer of electrons between the anode and cathode sites; this path is normally the substrate metal. The electrolyte is normally an aqueous solution around the substrate metal and contains ionic species capable of transferring charge between the anode and the cathode of the substrate metal. The anode is the location where the substrate metal corrodes and mass loss occurs. At the anode, metal atoms lose electrons and convert to metal ions which are drawn into the surrounding electrolyte. At the cathode, ionic species receive electrons from the substrate metal and convert them back to molecular form. Potential difference (or tendency to corrode) between the anode and cathode can result from many different conditions which include: variations in metal or alloy compositions; difference in amount of dissolved oxygen; presence of impurities on the substrate; ionic strength and/or constituents; temperature differences; etc.
Corrosion can be prevented, halted, or reduced by interrupting the transfer of electrons, by changing the chemistry at the anode or cathode, or by isolating the substrate from the electrolyte. Methods to achieve the prevention of corrosion include the use of barrier coatings or claddings, sacrificial coatings, corrosion inhibitors, cathode protection, and surface passivation. The barrier coatings or claddings include paints, organic coatings, ceramic and inorganic coatings, plastics, noble metal platings or claddings (such as nickel) and more.
Sacrificial coatings prevent corrosion by having a greater tendency to corrode than the metal they protect, thus converting the substrate metal to a more noble (non-corrosive) potential. Sacrificial coatings include zinc, aluminum, and magnesium metals and alloys applied as claddings, hot dip coatings, platings, or as fillers in primers and paints or other organic coatings. Corrosion inhibitors change the surface chemistry at the interface between the metal substrate and the electrolyte solution. This interface barrier may be formed by oxidizing the anode surface, precipitating a film or barrier layer that limits diffusion of ionic species between the bulk electrolyte and the substrate surface, or by adsorbing compounds which impart a hydrophobic film to the substrate metal surface. Cathodic protection of a substrate surface may be achieved by converting the entire surface of the substrate metal to a cathode through the use of sacrificial anodes or impressed electrical current. Surface passivation involves importing an oxide film to the substrate metal surface, thus preventing or reducing the tendency of the substrate metal to develop anode and cathode sites. Metallic substrates develop passive surfaces in specific environments or when exposed to solutions with specific pH ranges. For example, steel and iron substrates are naturally passive when exposed to aqueous solutions that have a pH of 8.5 or above. Aluminum also has a naturally passive surface due to an oxide that forms a tightly adherent oxide film limiting further exposure of oxygen to the metal substrate. Passive films, however, can be attacked and compromised by certain ionic species. In the case of iron and iron alloy materials, the naturally passive surface can be compromised by chloride ions and hydrogen ions, among others.
The previously mentioned mechanisms of corrosion protection have various drawbacks. Barrier coatings can be expensive and offer very little protection against corrosion if they are compromised, damaged mechanically, or have insufficient coverage. Sacrificial coatings have the potential of creating embrittlement of high strength steels due to the creation of monatomic hydrogen by-products from the corrosion reaction. The coatings may also be rapidly used up under certain accelerated corrosion conditions. Corrosion inhibitors are often expensive and some have been shown to be environmentally unfriendly or toxic. Many of these are available only as liquids making them inappropriate for certain applications as they function best in certain concentration ranges. Cathodic protection can be an expensive protection means that requires skilled professionals for its design and application. It is more readily applicable to new structures, but is difficult and/or expensive to install on an existing structure. Surface passivation has been used relatively little because it requires control of the environment around the substrate metal surface.
In nature, stable materials exist at their lowest form of energy. Iron typically exists as iron oxide ore. Mankind spends a tremendous amount of money refining and adding energy to the iron ore to create steel and other iron products with the necessary properties for fabrication of metal products and for construction of roads, bridges, buildings, and the like. The natural response of such products to the environment is to return to their lowest, most stable energy state, i.e., the iron oxide state. This corrosion process is accelerated when the products are exposed to corrosive constituents in the environment. Large amounts of time and money are expended annually in the use of coating materials to inhibit such corrosion.
Eight types of corrosion are defined by the National Society of Corrosion Engineers, namely: (1) General; (2) Localized; (3)Galvanic; (4) Environmental Cracking; (5) Erosion-Corrosion Cavitation and Fretting; (6) Intergranular; (7) Dealloying; and (8) High Temperature. General corrosion results from open exposure to corrosive conditions. Localized corrosion affects smaller portions of the metal surface than general corrosion but the rate of penetration may be very fast. Crevice corrosion is a form of localized corrosion resulting from corrosive exposure in a shielded location where oxygen depletion occurs. The oxygen depletion results in the development of acidic conditions which accelerate the corrosive loss of base metal. Electromotive corrosion is an accelerated form of localized corrosion due to stray electrical currents passing through an active corrosion cell.
General corrosion, crevice corrosion and electromotive corrosion are typically the kinds of corrosion of primary concern with iron and steel products. However, this invention is not only applicable in inhibiting those kinds of corrosion, but also in inhibiting other types of corrosion.
Galvanic corrosion occurs when two metals with different potentials or tendencies to corrode are in metal-to-metal contact. Fretting corrosion is wear or the damage that occurs at the interface of two contacting surfaces, at least one of which is metal, when they are subject to rubbing, i.e., minute slippage relative to each other. Intergranular corrosion is a selective or localized attack at or adjacent to grain boundaries without appreciable attack in the grain. Dealloying corrosion is a phenomenon associated with selective removal of one or more components from an alloy.
Crevice corrosion is a special form of corrosive action in that it typically may involve a bacterial or microbial attack accompanied by an environmental change in pH with anaerobic conditions. Microbes that promote corrosion of metals can be classed into five general groups: 1) acid producers which oxidize sulfur compounds to change sulfur compounds to sulfuric acid; 2) slime formers which aid in the production of anaerobic micro environments; 3) sulfate reducers that consume hydrogen and depolarize cathodic sites; 4) hydrogen feeders which feed on hydrocarbons; and 5) metal ion concentrators/oxidizers that work in conjunction with other microbes to create thick, bulky deposits to create concentration cells. Typically, more than one type of microbe is working at any one crevice corrosion site in a symbiotic relationship, fostering the growth of others. This form of attack is often hard to detect, since by definition initiation is often in the small crevices concealed in an anaerobic site. These sites often exhibit deep pitting, compromising the structural integrity of the base metals under attack.
Conditions that favor the promulgation of corrosive microbes include anaerobic environments, pH in the range of 0.5-10 (depending on the type of microorganism), and high concentrations of hydrocarbons upon which the microbes feed. Metal strands used in concrete structures can be ideal initiation sites. The strands used in such structures are typically of 1.times.7 high strength steel construction. Unfortunately, almost as soon as such a concrete structure is placed, cracking begins to occur. These cracks allow liquid water and other corrosive materials to penetrate the structure and thereby compromise the integrity of the structure. The cracks become not only ideal crevice corrosion sites, but also places for potential physical degradation of a structure by alternating freeze and thaw cycles with the hydraulic force of ice and liquid water breaking up the concrete.
The current cable market for post tensioned and pretensioned pre-stressed concrete is dominated by bare steel 1.times.7 strand. Strand is available with a fusion bonded epoxy coating with the surface of the epoxy containing an optional aluminum oxide grit to promote bonding with concrete (ASTM Specification A882/A882M). In addition, 1.times.7 strand containing grease and covered with polyethylene is also available for specific applications. For bridge stays and tower supports, conduits manufactured from plastic or steel materials such as galvanized steel or polyethylene are currently typically used as containment for individual strands or multiple bundles of strands. The conduits are commonly filled with grout to strengthen the assembly and to fill in the interstices. The alkaline nature of the grout passivates the surface of the steel from corrosion. Strand and wire rope for the automotive industry and other general purpose uses currently typically have a zinc or zinc alloy hot dip coating and may be covered with a number of thermoplastic coatings. The zinc or zinc alloy coatings protect the surface of the steel from corrosion galvanically due to its sacrificial nature (it corrodes preferentially to steel).
Organic plastic materials have also been used to fill the interstices between the wires on some cable products. Ceramic based, corrosion resistant coatings have been developed and are currently in use in automotive parking brake cable strand products. However, ceramic coatings are not usually suitable for wire rope products because the abrasiveness of such coatings reduces fatigue life of the wire rope.
The use of bare strand products causes problems when exposed to corrosive environments. The strand surface is not protected from corrosive constituents in the environment. Strand stored to be placed in casting beds (forms that are reused time and time again for making of structural elements of precast concrete) often show signs of initial red rust before being surrounded by the protective encasement of concrete. In the case of strand used as a prestressor for concrete materials, corrosion can occur when corrosive constituents such as salts from deicing compounds, admixtures to the concrete, or marine environment migrate to the surface of the strand. Initially, the pH in concrete is near 12.0. Corrosiveness increases as the concrete pH drops or when carbonation occurs. Due to the chemical reactions incurred during the cure cycle of the concrete mixture, there are zones of cure that striate and promote cracking at the surface of the curing mixture. Depending upon the cure conditions, the types of admixtures and aggregates, and the surrounding physical conditions, this cracking may or may not be readily visible to the naked eye. The expansive forces caused by the development of corrosion on the steel strand or other steel reinforcement can also crack the concrete. Whatever their origin, cracks can accelerate the degradation of the concrete and also the degradation of prestressing strand (by exposing the steel), necessitating repair or replacement of the concrete member to avoid failure of the structure. Bare strand has no protection against corrosion except for the surrounding concrete. Rehabilitation projects often use sealers on cracked concrete to lessen the chance that liquid water or other corrosive elements will enter the cracks and accelerate the degradation process. Typically these sealers only provide a physical barrier and do not address the underlying steel components that are the subject of the corrosive attack.
Epoxy coated strand, the only current commercial alternative to bare strand for pretension prestressing applications, has a number of limitations. The epoxy coating effectively offers only barrier protection against corrosive constituents. When the barrier has been compromised due to coating imperfections or field handling, corrosive constituents can get to the strand wires and cause local corrosion to develop. The corrosive constituents can then migrate beneath the epoxy coating, developing crevice corrosion cells and cause delamination of the epoxy coating from the surface of the strand wires. Crevice corrosion can cause rapid failure of strand wires as well as a reduction of the pH in the corrosion cell area. The epoxy coating has good overall chemical resistance, but can be attacked by oxidizing materials such as chlorine, fluorine, and hypochlorite materials. Sodium hypochlorite is commonly used as a water treatment additive and for bleaching in the pulp and paper industries. The hypochlorite ion may also be formed in alkaline environments (such as concrete) when chloride ions or chlorine is present.
Construction of precast members using epoxy coated strand requires the use of special jaws on the strand during the tensioning process. Even with the special jaws, slippage sometimes occurs, causing removal of the epoxy coating and adding difficulty to the construction of the precast members. Jaws that are typically reused 60-70 times for bare strand often require cleaning after only two or three uses with epoxy coated strand.
It is common practice for post tension cables to be encased with hydrocarbon greases and sheathed with a polymeric coating that excludes oxygen. Strand and wire rope products filled with grease are typically limited to applications where high bond strength is not required (post tension applications) and bridges. Grease has been shown to offer good corrosion protection when present in excess; however, bare areas can occur due to rubbing. Greases may be expelled during periods of high temperatures, and many oils and greases become acidic with the passage of time, increasing the threat of corrosion. This acidic condition speeds the ionic exchange between the surrounding electrolyte and the metal substrate. The grouting of strand tendons can lead to air pockets or voids between the tendons where corrosive materials can collect. Some admixtures commonly used in grout formulations are corrosive with respect to steel reinforcement.
The interstices between the individual wires that form cables or strands frequently show the first place of attack. A paper in 1985 described the process of a bio-film community as follows: (1) iron-oxidizing bacteria infiltrate metal surfaces and put down "roots" to anchor a community; (2) slime formers and fungi are attracted to this site because of the nutrient availability and/or protection; (3) sulfate reducing bacteria thrive in this anaerobic layer producing copious amounts of corrosive hydrogen sulfide gas; (4) a layered aerobic/anaerobic stratification forms typically with the first layer of bio-film protecting an underlying anaerobic community.
Electromotive corrosion, like crevice corrosion, is also an accelerated form of corrosion which may be due to stray electrical currents from nearby cathodic protection systems or to static electrical currents generated from frictional contact as for example, between automobile tires and driving surfaces, as well as gear and bearing friction (and subsequently discharged through the automotive parking brake cables).
Many different types of corrosive environments exist: Marine, Rust Belt, Water Treatment Facilities, Power Plants, Pulp and Paper Mills, and Chemical Process Plants. Natural corrosives such as salts from marine environments, and man-made corrosives such as acid pollutants, corrode infrastructures and metal products. Structures requiring protection from corrosive constituents include roads, bridges, dams, parking garages, and piers. In that regard, people everywhere are increasing demands for snow and ice free roads, bridges, and parking garages through the use of deicing salts; unfortunately, these salts further contribute to the deterioration of infrastructure. Metal products requiring protection from corrosion constituents include metal parts of boats and marine equipment, automobiles and aircraft.
Considerable effort has been expended by the construction and automobile industries, among others, to delay and reduce the rate of corrosion of steel. Such corrosion can result in failure not only of the steel, but also the elements and structures the steel supports. For example, corrosion of reinforcing steel members in concrete is known to result in deterioration of the concrete. Such deterioration is believed to be due in large part to the fact that the corrosion products tend to occupy a greater volume of space than the original steel, resulting in stresses on the surrounding concrete material. Structural distress may also occur due to a reduced cross-sectional area of the steel or to a loss of bond between the steel and the concrete.
In industrial applications, metal corrosion can be accelerated by several factors such as the infiltration of oxygen and moisture ("general corrosion") and the presence of stray electrical currents ("electromotive corrosion"). Metal corrosion can also be accelerated by bacterial attack in highly acidic (i.e., pH&lt;2.0), anaerobic environments on the metal surface ("microbiological induced corrosion").
One well-known method for preventing steel corrosion is galvanization. In particular, zinc and zinc alloys together with thermoplastic coverings are commonly used to coat strands and wire rope in many industries. Zinc and its alloys are known to protect the surface of steel sacrificially, in that zinc corrodes preferentially to steel. However, a major disadvantage of this treatment is that the zinc coating provides only temporary protection of the base metal. Also, the coating may corrode unevenly, jeopardizing the integrity of the underlying metal. Zinc electroplating processes used to galvanize ferrous substrates often lead to hydrogen embrittlement of steel products, notably the high strength, highly stressed steels. In addition, the corrosion of any galvanic zinc coating (electroplate, mechanical plate, cladding, thermal spray, hot dip, or zinc filled coating) can cause hydrogen embrittlement of high strength, highly stressed steels such as reinforcing strand used in concrete and bridge stays.
For the above reasons, many industries have begun to investigate alternative corrosion treatments. For example, most such treatments which have been developed for wire products involve coating or encapsulating the base metal with various compounds such as plastics, ceramics, epoxy resins, greases, and other hydrocarbon-based substances. These substances may be applied either on the outside of the wire ropes or strands or in the interstices of the ropes or strands. However, these treatments only provide a physical barrier to corrosive elements such as moisture and oxygen, and they do not addrese the corrosion of the steel itself. Furthermore, even with such applications, the steel is still vulnerable to crevice corrosion, since highly acidic, anaerobic enviroments will often appear on the metal surface, thereby promoting bacterial attack.
Current industry standards for corrosion prevention center around the use of heavy metals (chromium, nickel, lead, cadmium, copper, mercury, barium, etc.) or heavy metal compounds to passivate or provide a barrier to inhibit, or galvanically sacrifice, and thereby protect the substrate metal beneath. The introduction of these materials into the environment, however, can lead to serious health consequences as well as substantial costs to contain or separate the materials or clean up environmental contamination. Dealing with corrosion, accordingly, is a continuing problem and better systems for preventing corrosion are still needed.